1. Field of the Invention
The present invention relates to an audio apparatus that is adapted for generation of low tones by means of a small loudspeaker, for example.
2. Description of Prior Art
Various conventional techniques are available and have been proposed for generating low tones or bass tones using small loudspeakers. For example, FIG. 4 shows an example of a loudspeaker driving circuit constructed by combining Helmholtz's resonance technique and negative driving technique. According to this circuit, low tones can be efficiently produced even when a small loudspeaker is used.
The circuit shown in FIG. 4 will now be described briefly. In FIG. 4, reference numeral 1 denotes an inversion amplification circuit 1 that includes resistors R1 and R2 for determining an amplification rate or gain, and a differential amplifier 2. A loudspeaker SP and a resistor Rs are inserted in series between an output terminal of the differential amplifier 2 and the ground. The junction point of the loudspeaker SP and the resistor Rs is connected via a feedback amplifier 5 to a positive input terminal of the differential amplifier 2. In this case, an amplification rate or gain of the feedback amplifier 5 is set to A (a fixed value). Reference numeral 6 denotes an adder, via which an input signal Si is inputted to the inversion amplification circuit 1. Further, an attenuator 10 and a level control circuit 11 together constitute a limiting circuit 12 that is inserted between the output terminal of the inversion amplification circuit 1 and the adder 6. Such a type of the audio apparatus is disclosed in Japanese Unexamined Patent Publication No. Hei 1-302997 and No. Hei 1-302998. The U.S. Pat. Nos. 4,987,564 and 4,989,187 are based on these Japanese patents, and all their contents are herein incorporated into the specification by referring to the patent numbers.
In the circuit of FIG. 4, output impedance Zo of the differential amplification circuit 1 is represented as:
Zo=Rs·(1−((R1+R2)/R1)·A), and it is understood that the impedance Zo is given as a negative resistance.
Curves C1 to C3 in FIG. 2 represent voltage amplitude-frequency characteristics measured when the loudspeaker SP of the above-described circuit is in the loaded state. From the curve C1 to the curve C3 there is an increase in a level of the input voltage. The curves C1, C2 and C3 represent the frequency characteristics measured when the input voltage level is 0 dB, 10 dB and 20 dB, respectively. Level L1 in Fig. 2 indicates a limiting value for the voltage amplitude.
As for the curve C3, there is a portion in the low band area that exceeds the limiting level L1 (see a shaded portion in FIG. 2), and a clip occurs. That is, waveform components of the input signal that are present in this band area are distorted and higher order harmonics are generated. To avoid such a distortion, a limiter must be employed to prevent the voltage amplitude from reaching the level L1.
The amplitude-frequency characteristic of the circuit of FIG. 4 has two peaks as shown in FIG. 2, and the amplitude level on the low band side (around 30 Hz) is especially high. This characteristic is preferable when a low tone is to be outputted; however, the peaks on the low tone side must be limited in order to prevent the voltage amplitude from exceeding the limiting level L1. Therefore, the limiter must be activated even when the input voltage is still comparatively low. In the example of FIG. 2, the limiter must be activated when the input level exceeds 10 dB (corresponding to the curve C2). The limiting circuit 12 must be prepared so that the circuit can be activated in an early stage in order to cope with a possible rise in the input voltage.
However, if the limiter is applied at even a low level as is described above, optimal characteristics for the reproduction of low tones are not expected, and the full inherent performance of the audio system cannot be obtained.
As is described above, the conventional method is designed for applying the limiter while the voltage level is still low. In such a method, if the peak of the frequency characteristic of the audio apparatus is high, and if the input voltage level is increased (namely, the volume control is raised), the original frequency characteristic cannot be performed. Further, the limiter becomes too effective to the peak of the frequency characteristic as the volume is increased, hence unnatural feeling is created in the sound generation.